1. Field of the Invention
This invention relates to a method and apparatus for reconnecting a motor to a motor drive. In particular, the present invention relates to a method and apparatus for reconnecting a rotating motor to a motor drive in which one or more voltage commands produced by the motor drive are used to provide an indication of the speed and direction of rotation of the motor. The present invention also relates to a method and apparatus for reconnecting a rotating motor to a motor drive in which a motor is excited using positive feedback so that a voltage induced in the motor may be measured more accurately.
2. Description of Related Art
During normal operation of a motor, the motor may become disconnected from a motor drive which drives the motor (meaning that the power supply to the motor is interrupted, not necessarily that the electrical connection between the motor drive and the motor is interrupted). There are a variety of reasons why this might occur. For example, there may be a sudden temporary power loss at the power source that supplies power to the motor and motor drive. Alternatively, it may simply be the case that there are times when it is not necessary to operate the motor, and so power is not supplied to the motor during these times.
When a motor drive becomes disconnected from a motor, it eventually becomes necessary to reconnect the motor drive to the motor. In order to perform the reconnection, it is necessary for the motor drive to "catch" the motor, that is, to determine the speed (and preferably also the direction) of rotation of the motor, before the motor drive is reconnected to the motor. The fact that the motor drive is disconnected from the motor does not prevent the motor from continuing to rotate. For example, if the motor is used in conjunction with a fan in an air conditioning system, a draft in the air conditioning system may drive the motor at an unknown speed and in an unknown direction. Similarly, if the motor is used in a conveyor system, the force of gravity acting on the motor by way of the conveyed articles and friction may drive the motor at an unknown speed and in an unknown direction.
If the speed and direction of the motor are not determined before reconnection, then the motor drive must assume an initial speed of zero when reconnecting to the motor. This often results in severe transients due to a high initial slip and due to the effects of the motor-induced back EMF on the current regulators or limiters in the motor drive. The transients are especially severe when the initial motor speed is high and when the motor is rotating in a reverse direction as compared to that commanded by the motor drive. If the current control circuitry or current limiting circuitry of the motor drive is not fast enough, the motor drive can fault due to an overcurrent condition.
In order to avoid undesirable transients and overcurrent conditions upon reconnection of the motor drive, various approaches have been developed for catching a motor. By catching the motor, the frequency and phase of the voltage supplied by the motor drive may be synchronized to the speed and direction of rotation of the motor when the motor drive is reconnected. This reduces the initial slip and therefore reduces transients upon reconnection.
According to a first approach, a mechanical sensor is utilized. The mechanical sensor, such as an encoder or resolver, is used to directly measure the speed and direction of rotation of the motor. The disadvantage of this approach is that it requires additional hardware and therefore increases the cost of the system.
According to a second approach, voltage feedback is utilized. The frequency and phase of the back EMF produced by the motor provides a direct indication of the speed and direction of rotation. One disadvantage of the voltage feedback approaches which have been provided thus far is that they require additional hardware (i.e., the hardware necessary to perform the voltage measurement).
Another disadvantage of most voltage feedback approaches provided thus far is that they do not work satisfactorily in conjunction with induction motors, especially small induction motors (less than five or ten horsepower). Unlike DC motors and synchronous motors, which always produce a back EMF voltage when driven mechanically, induction motors do not produce a back EMF unless special measures are taken to excite a field in the induction motor. Although a back EMF can be induced in an induction motor, for example, by applying current pulses to the motor as the motor rotates, this does not work well if the motor is large (because it is very difficult to excite the motor in a short amount of time) or small (because the induced voltage to decays too rapidly to be accurately measured). Alternatively, the remanence voltage of the induction motor can also be utilized for voltage feedback. However, this alternative is only usable in conjunction with large induction motors, because the amplitude of the remanence voltage produced by small induction motors is not sufficient to permit the speed to be accurately determined.
Induction motors, and especially small induction motors, therefore present a unique problem. Unlike all other types of motors, induction motors do not automatically produce a back EMF voltage. Therefore, the preferred approach for determining the speed of an induction motor is usually mechanical feedback. However, small induction motors tend to be more cost-sensitive and the increase in cost associated with the use of mechanical feedback is often unacceptable.
According to a third approach, frequency sweeping is utilized. Frequency sweeping is used primarily in conjunction with induction motors. According to this approach, the motor is excited with a small current having a frequency which is swept downward from a maximum frequency. If the excitation frequency is higher than the speed of the motor, then the motor acts as a motor; otherwise, the motor acts as a generator. The speed of the motor can therefore be detected by detecting the reversal in power which occurs when the excitation frequency crosses the speed of the motor.
The disadvantage of this approach is that the frequency sweeping can take several seconds to complete. In many applications, it is desirable to reconnect to the motor drive as quickly as possible, such as in the range of 300-500 milliseconds. For example, when there is a temporary power loss to a motor which drives a conveyor belt, it is desirable for operation to continue as smoothly as possible without jostling the articles on the conveyor belt. Hence, in many applications, the several seconds required to perform the reconnection is unacceptable.
In short, although various approaches have been developed for determining the speed and direction of rotation of a motor before reconnection, these approaches all suffer from one or more of the above-mentioned drawbacks. It would be highly advantageous if an approach could be provided which does not suffer these drawbacks. Namely, what is needed is an approach which does not require additional hardware, which is able to reconnect a motor in a short amount of time, and which is usable in conjunction with all types of motors, and especially in conjunction with induction motors.